Anticholinesterases

The action of all the neuromuscular blocking drugs wears off spontaneously with time, but this is not always clinically appropriate. In patients who require reversal of neuromuscular blocking drugs, an anticholinesterase is given. This inhibits the action of the enzyme acetylcholinesterase, resulting in an increase in the concentration of acetyl-choline at the neuromuscular junction (nicotinic effect). The speed of recovery will depend upon the intensity of block when reversal is attempted—the more intense the block the slower the reversal. Anticholinesterases cannot be used to reverse very intense block, for example if given soon after the administration of a relaxant (no response to a 'train-of-four' sequence — see below).

Anticholinesterases also function at parasympa-thetic nerve endings (muscarinic effect), causing bradycardia, spasm of the bowel, bladder and bronchi, increased bronchial secretions, etc. Therefore they are always administered with a suitable dose of atropine or glycopyrrolate to block the unwanted muscarinic effects.

The most commonly used anticholinesterase is neostigmine:

• A fixed dose of 2.5 mg intravenously is used in adults.

• Its maximal effect is seen after approximately 5 mins and lasts for 20-30mins.

• It is given concurrently with either atropine 1.2mg or glycopyrrolate 0.5 mg.

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